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The famous Pole Star lies less than one degree from the Northern Celestial Pole,
and so always lies north from an Earth-bound observer's point of view. Its name comes
from Latin, Stella Polaris, meaning simply 'Pole Star'. Its alternative
and much rarer name, Cynosura, comes from the Greek for 'tail of the dog'.
As the Earth turns on its axis, the stars in the sky seem to turn around the
Northern Celestial Pole. This image shows the paths traced by the
stars as the swarm around the Pole over
a period of eight hours. As the nearest bright star to the
Pole, Polaris appears as the small
bright crescent in the centre of the image.
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Physically, Polaris is very massive star of the 'F'
(bright yellow) classification.
It is particularly notable as being one of the nearest Cepheid variables to
Earth; it is less then half the distance of Delta Cephei itself.
This is important because Polaris' four-day cycle of swelling and contraction is
directly related to its mass, which is in turn related to its luminosity. This means
that we can calculate the star's
absolute magnitude with some certainty, and comparing
this with its observed apparent magnitude, we can compute its distance.
Polaris' mean absolute magnitude is -3.50, and its mean
apparent magnitude is +2.11, which gives us a distance of just
over 431 light years.
The Galactic position and direction of Polaris relative to Earth's Sun. Note that, at this
extreme scale, the two stars are effectively in the same place.
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